Photochemical Electron‐Transfer Generation of Arylthiirane Radical Cations with Tetranitromethane and Chloranil — Some Novel Observations

Abstract

The radical cations from 2,2,3,3‐tetraphenylthiirane (1a), 2,2‐bis(4‐methoxyphenyl)‐3,3‐diphenylthiirane (1b), and trans‐2,3‐diphenylthiirane (1c) have been generated by photoinduced electron transfer (PET) reactions with tetranitromethane [C(NO2)4] and chloranil (CA). A charge‐transfer complexe (CTC) absorption is observed by UV/Vis spectroscopy between thiiranes (1) and C(NO2)4. On the other hand, quenching studies with azulene suggest that the ET reaction occurs between thiiranes and the triplet CA (3CA). The photochemical reaction of the CTC between thiiranes 1 and C(NO2)4 yields mainly the corresponding alkenes from the fragmentation of the radical‐cation intermediate 1·+, together with the products derived from nitration on the phenyl rings. However, oxygen transfer to afford the sulfoxides is not found. A marked solvent effect is observed in this reaction, with cage coupling favored in CH2Cl2 (nitration derivatives as primary products) and non‐cage coupling observed in CH3CN (the alkene as the primary product). The PET reactions between 1a–b and CA, in the presence of CH3OH (or another possible oxygen‐centered nucleophile), give the ketone derivatives through ring opening, followed by oxidative cleavage. Conversely, under the same experimental conditions, the thiirane 1c affords only trans‐stilbene 2c. This different behavior is ascribed to a different spin density in the corresponding singly occupied molecular orbital (SOMO) of the radical cation. For 1c+·, the spin density is concentrated at the sulfur atom, whereas for 1a·+ and 1b·+, the charge is distributed onto the aromatic rings. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

An Unusual Pathway to Cyclobutane Formation via Desulfurative Intramolecular Photocycloaddition of an Enone Benzothiazoline Pair

“…However, the trans stereochemical relationship between the radical centers in 12 precludes carbon−sulfur bond formation but could still lead to the formation of episulfide 13 . Extrusion of sulfur from 13 can then lead to the formation of enecarbamate 14 (R = COOMe) . Alternatively, Grob-like fragmentation of 10 / 12 could lead to the direct formation of 14 with loss of elemental sulfur.…”